1. Field of the Invention
This invention relates to double-sided copper foil substrates, and more particularly, to a composite double-sided copper foil substrate used for flexible printed circuit boards.
2. Description of Related Art
Recently, electronic systems are developed with a trend of compact size, high thermal resistance, versatility, high density, high reliability and low cost. Thus, the selection of a substrate is a very important factor.
A copper foil substrate used for manufacturing a flexible printed circuit board is classified into a single-sided copper foil substrate and a double-sided copper foil substrate, wherein the single-sided foil copper substrate or the double-sided copper foil substrate is selected in accordance with actual need. Regarding the manufacture process, the copper foil substrate is cut into a specific size and then a patterning process is executed. Compared to the single-sided copper foil substrate, the double-sided copper foil substrate can be formed with more circuits within a limited space, and thus the double-sided copper foil substrate is a primary intra substrate of the multi-layer board.
Currently, the structure of double-sided copper foil substrate 100 applied to the flexible printed circuit board, as shown in FIG. 1, is mainly manufactured by providing thermoplastic polyimide (TPI) films 102 on each of top and bottom surfaces of a thermosetting polyimide film 101 respectively and subsequently providing copper foils 103 on the surfaces opposing the thermosetting polyimide film 101 of each of the two polyimide films 102 such that a patterning process can be performed on the copper foils 103. However, since the advantages of a specific thermosetting polyimide film usage include having source limitation by international manufacturers such as American or Japanese manufacturers, the soaring cost and the fabrication facilities for the double-sided substrate manufactured by laminating, having problems in the yield rate thereof, are expensive, the cost of the flexible printed circuit increases such that the average cost of the resultant flexible printed board increases correspondingly
Regarding another structure of double-sided copper foil substrate 200 applied to the flexible printed circuit board, as shown in FIG. 2, is manufactured by laminating copper foils 203 on the exterior sides of the combined polyimide layers 201 and 202 having different thermal expansion coefficients such that a patterning process is performed on the copper foils 203. However, since the thermal expansion coefficients of the polyimide layers 201 and 202 are asymmetric, a non-uniform stress may cause warpage and the bending R angle of less than 0.8 mm can not be achieved.
Therefore, there is still a need for a double-sided copper foil substrate having a greater number of bending cycles and siding cycles and the bended R angle is less than 0.8 mm.